1. Field of the Invention
The present invention relates to devices for initiating crack propagation in frangible materials. More particularly, the present invention relates to aircraft canopy removing devices. Even more particularly the present invention relates to devices for initiating crack propagation in aircraft canopies.
2. Background Information
Through-the-canopy ejection is commonly employed both as a primary method and as a back-up method of emergency aircrew ejection. As a primary method, through-the-canopy ejection is often selected during design to reduce the total time required for aircrew ejection. This is accomplished by reducing the time required for all subsystems to function, from aircrew recognition of the need to initiate escape until the aircrew are clear of the aircraft. It is also employed as a primary mode to reduce the overall system complexity in order to enhance the overall system reliability by eliminating a serial-step type process which could preclude ejection if any of the serial steps failed. Often through-the-canopy ejection is selected as a back-up to jettisoned-canopy ejection, even at some increased risk of injury or of life support equipment damage, simply to preclude the failure of the canopy jettisoning system from causing the death of the affected aircrew.
Successful through-the-canopy ejection requires that the penetration of the canopy be smooth, i.e., not resulting in any rapid and/or large velocity changes which would induce separation of the ejectee and seat and an increase in the internal pressure of the boost catapult. This combination can result in a rapid seat acceleration upon breaking through the canopy with subsequent injurious levels of "overshoot G" when the seat catches up to the ejectee. Successful through-the-canopy ejection also requires that a seat and its occupant (ejectee) actually break through the cockpit canopy. The seat must not be stalled or delayed so long that the ballistic gas pressure dissipates through leakage around seals and through heat loss through the surface. Such an occurrence could yield fatal results for the ejecting aircrew.
In the past, several proposals for canopy removal have been put forth, however, these proposals all involve complex methods for removing the canopy. One such proposal, disclosed in U.S. Pat. No. 3,542,319 issued to Duncan et al., teaches the use of a catapulting canopy breaker which is attached to telescoping inner and outer tube members. In Duncan, a system using fluid pressure is disclosed to drive the canopy breaker upward so that it impacts and shatters the canopy before the ejecting aircrew member is jettisoned from the aircraft. This method requires using scarce cockpit space to locate the telescoping tubes for the canopy breaker. Moreover, the Duncan canopy breaker is not easily adapted for differing canopy and cockpit configurations.
Other somewhat different approaches involve the use of explosively actuated devices for explosively unlatching the canopy from the aircraft, for explosively operating a movable breaker member which could shatter the canopy, or for directly shattering the canopy. One such approach is represented by U.S. Pat. No. 4,275,858 issued to Bolton et al. In Bolton, fusible conductors are disclosed as being embedded in intimate contact with the canopy material and further connected to a source of electrical power such that a high-energy pulse of current is generated sufficient to fuse the connector and initiate crack propagation in the canopy. These types of proposals, however, necessarily involve the provision of separate explosive charge means and a mechanism for energizing the same. The overall ejection assembly is thereby increased in cost, weight and system complexity, the latter of which markedly affects the degree of maintainability of the system. Moreover, these types of approaches are not readily transferrable from one aircraft configuration to another. This lack of adaptability from one cockpit design to another demands that different systems be employed. Each system would likely have varying service requirements. Maintenance crews would, therefore, need to learn specific requirements for several different configurations. The provision of a more uniform, but less complex canopy breaker system would streamline operations and maintenance for differing canopy and cockpit configurations.
Given a large enough run, for example, 1" to 1-1/2", and small area incompressible contact points, most ejection seats can smoothly penetrate most canopies. One problem adversely affecting through-the-canopy ejection, however, occurs when, due to logistical or other considerations, an ejection seat's contact areas cannot be designed for a specific canopy curvature configuration to assure minimal area initial contact. That is, the instantaneous initial contact area is too great, thereby reducing the localized stress imposed upon the cockpit canopy to levels below its failure thresholds. Another problem degrading through-the-canopy capability is that of the increased plasticity of canopy glazing materials with elevated temperatures. Such conditions potentially occur under certain climatic conditions or under certain flight conditions which induce strong aerodynamic heating of the canopy glazing material. Under these latter conditions, the canopy glazing material in the area of seat headrest impact typically molds itself around the headbox and other protuberances of the rising seat while the sides of the canopy glazing stretch and deform geometrically. The seat's energy is absorbed and the seat is slowed significantly, even to the extent of stopping over a moderate distance of travel. As noted in the previous paragraph, such slowing of the ejection seat, should it last sufficiently long, can rob most catapults of their energy and result in the system not ejecting the aircrew from the disabled aircraft.
Due to the potential that during parachute deployment, the parachute canopy and/or the parachute suspension lines might contact areas of the headrest, one solution, that of using very sharp ("knife sharp") edge canopy breakers, often is denied the ejection seat system designer. The rational for this is a fear that more ejectees will be endangered by serious parachute damage than might otherwise possibly be placed at risk of non-ejection or of overshoot-G-induced spinal injury if a blunt area canopy breaker were chosen instead. Even were concerns regarding parachute subsystem integrity non-existent, the placement of very sharp ("knife edges") on the top of an ejection seat would entail severe risks to aircraft maintenance personnel which would make their adoption most unlikely. Performance of some cockpit maintenance tasks often requires that maintenance personnel reach over and behind ejection seats in ways requiring their torsos to heavily contact the tops of ejection seats. Sharp objects, especially sharp penetrators, on the headrests could result in many injuries and could degrade aircraft systems maintenance and, thereby, aircraft operational readiness and performance as maintenance personnel attempt to avoid injuries.